The invention relates to a diffusion tip for a fuel nozzle for use in gas turbines. More particularly, the invention relates to a diffusion tip configuration and adaptations for cooling the same.
In a gas turbine, fuel nozzles are used to mix air and fuel for later combustion downstream. A diffusion mode is used for stable combustion during start up until premixed mode can be used to reduce NOx emissions. The diffusion tip of the nozzle must provide a mechanism for generating the diffusion flame during start up and remain cool enough to resist damage from hot combustion gases during premixed mode. Current designs use air diverted from the main path to cool the diffusion tip resulting in an uncertain air proportion of cooling versus main flow and a complicated flow path.
More specifically, a conventional diffusion tip 10 is illustrated in FIG. 1. As illustrated therein, the current design splits curtain air 12 into burner tube cooling air 14, diffusion air 16 and shower head air 18. As understood from the phantom line passage depiction at 20, the diffusion purge does not flow to the shower head portion 22. The flow split and therefore effective cooling in the three circuits 14, 16, 18 can vary based on input conditions and the cooling of the tip (from shower head air effusion cooling) cannot be independently modified. As illustrated, the configuration of FIG. 1 uses a plurality of holes 24 to accomplish the effusion cooling in the diffusion tip. For instances of high thermal and/or structural loading, these holes can act as stress intensification sites, reducing life to crack initiation. In addition, these holes may allow combustion gas entry into the diffusion cooling circuit if the pressure of the combustion gas is locally higher than the pressure in the diffusion cooling circuit.